Demand for compound semiconductor materials whose properties can be tuned by adjusting the compositions of their constituent parts has been increasing with the need for electronic and optoelectronic devices that have superior performance characteristics for various application requirements at low cost. Compound semiconductors, such as those from group III and V, or from group IV of the Periodic Table of elements, have been, and continue to be the mainstay of modern electronic and optoelectronic technology. Appropriate compositions of III-V compound semiconductors comprised of (In,Ga,Al)(As,Sb,P), for example, grown on binary GaAs, GaSb, or InP base substrates are widely used in the manufacture of transistor, sensor, electronic memory, laser and light-emitting diode chips utilized in high-performance instruments such as radar systems, radios, and cell phones, to name only a few of the key applications of these materials. These same materials are also widely used to make semiconductor photodetectors that are widely used in high bandwidth fiber-optic communication systems, optical disk systems, such as compact disc, and digital versatile disc systems, specialty illumination systems, and advanced photovoltaic solar cells. In another example that illustrates the importance of tunable properties of compound semiconductors, various compositions of silicon-germanium (SiGe) grown on Si play a critical role in WiFi and wireless electronic systems. A fundamental requirement for being able to prepare thin films of semiconductor layers for device structures is the availability of base substrates on which new films can be grown. The thin films can generally be doped lightly N-type (1016 cm−3) or heavily N-type (≧9×1018 cm−3); they may also be doped lightly or heavily P-type. A device structure grown on a substrate is defined to be a sequence of thin film layers whose composition, thickness, doping type and levels, electrical, and optical properties are selected or designed to allow the ensemble to perform a specific function. The substrate material must generally be structurally and chemically compatible with the thin films to be deposited on it. Furthermore, the in-plane periodicity of the atoms of the base substrate must match that of the desired over-layer thin films to be deposited on the base. Substrates must provide appropriate initiation or seeding surfaces on which new growth can occur. For common III-V compound semiconductors such as (In,Al,Al)(As,Sb,P), it has been possible to satisfy this requirement by using binary GaAs, GaSb, or an InP base substrates. Even though the in-plane atomic periodicity of these substrates do not match all compositions of (In,Ga,Al)(As,Sb,P) one can choose appropriate percentage compositions of (In,Ga,Al)(As,Sb,P) that can be approximately matched to the binary GaAs, GaSb, or InP substrates so that there is an acceptable atomic registry between the base substrate and the thin films on top. When this atomic registry is grossly mismatched, say by more than 3%, a defect-free thick film of the over-layer material cannot be uniformly grown on the substrate. The thickness depends on the particular composition of the over-layer desired. It can range from as thin as a few nanometers to as thick as hundreds of nanometers. The mismatch, often referred to as a lattice-mismatch, can lead to atomic dislocations that interrupt the pristine periodicity and order of the atoms in the thin film over-layer. The major consequence of this mismatch and the resulting dislocations is an impairment of the electronic and optical properties of the thin films. This can sometimes be mitigated by clever techniques such as deposition of intermediate layers with special temperature cycling steps (as discussed in Shibata's U.S. Pat. No. 7,771,849) or deposition of compositionally graded (In,Ga,Al)(As,Sb,P) layers that progressively increase the percentage of one or more of the constituents of the material until the desired composition and corresponding lattice constant of the over-layer is reached. Another common mitigation technique is to deposit extremely thin (few nanometer) layers of two compositions of the over-layer in an alternating periodic fashion, thus forming what is called a superlattice. Such a superlattice structure alternately causes the films to be in compression and tension in such as manner that the mismatch is accommodated elastically without dislocations or defects. Both mitigation approaches only work when the mismatch is small or when only thin (sub-micrometer) over-layers are desired.
The class of III-V compounds comprised of two, three, or four constituent elements of (In,Ga,Al)(As,Sb, P), with at least one element from group III and one group V, have band gaps that range from a low 0.17 eV for InSb to a high of 2.45 eV for AlP. For certain applications, such as in light-emitting diodes that operate in the ultraviolet, blue, or green regions of the spectrum, the energy band gap value of compound semiconductors must be larger than 2.45 eV. The common class of compound semiconductors discussed previously is therefore not suitable for light-emission at wavelengths below red (<620 nm). However, another class of III-V compound semiconductors that is comprised of (In,Ga,Al)N can be used. This class of materials posses band gaps that stretch from 0.67 to 6.04 eV. These semiconductors are very useful for a number of applications in electronics and optoelectronics; however, they are challenging to produce in pristine thin film form. This is largely due to lack of a suitable base substrate for the initial thin film epitaxial process. Until recently, there were no chemically compatible binary nitride substrates. However, GaN and AlN binary substrates have now become available. Even with the availability of these substrates, it is still challenging to produce many desirable and useful structures for electronic or optoelectronic devices. Almost all electronic and optoelectronic device structures require multiple layers of thin films to be deposited one-on-top-of-the-other on the base substrates. The majority of films used for devices and composed from compound semiconductors usually differ in composition (and hence lattice constants) from the substrates on which they are deposited; this inevitably leads to lattice-mismatches originating from the substrates, leading to dislocations and defects that impair crystalline quality and degrade the properties of the over-layer films. The most common substrates currently used for epitaxial synthesis of any composition of (In,Ga,Al)N films include α-Al2O3 (otherwise known as sapphire), SiC, GaN, and AlN. The binary compounds of SiC, GaN, and AlN are used in their bulk form. In this form, they can never be lattice-matched to the majority of ternary or quaternary compound semiconductor layers or structures that are most often grown on them. Prior to the advent of bulk binary nitride substrates, templates consisting of sapphire on which thin over-layers of GaN or AlN were grown, were used as base substrates. These templates can also never be lattice-matched to films grown on top of them because most device films are not comprised of binary materials and usually have lattice constants that are different from the binary templates on the sapphire. The major benefit of using bulk binary nitride substrates (GaN or AlN) is that they are chemically, thermally, and structurally compatible with device structures (which are constructed from a specific sequence of epitaxially grown thin films of various compositions of binary, ternary or quaternary films) that are grown on top of them. However, the lattice constant of any of the bulk binary nitride substrates or SiC can never be matched to that of any device structures grown on top of them for reasons stated in the foregoing. The lattice constant incompatibility has dogged the overarching goal of producing the highest performance or quality nitride device structures.
The need for substrates that are lattice-matched to the materials used in device structures grown on top of them is imperative because the mismatches constrain the diversity of compositions of compound semiconductor films that can be grown for devices of specific designs. This limitation manifests itself in many different forms. Often, because of the need to mitigate lattice-mismatch-induced dislocations and defects, epitaxial films are deliberately grown with non-optimal compositions or thicknesses. The non-optimality commonly leads to performance inefficiencies, for example, in lasers and light-emitting diodes that either emit much less optical power than desired or diodes that emit at wavelengths of less desirable colors. For electronic devices, lattice mismatches originating from base substrates degrade electron transport properties, thus limiting electron mobilities and hence high-frequency performance or switching speeds. The lack of ideal base substrates for epitaxial synthesis of (In,Ga,Al)N compound semiconductor films and device structures places a severe constraint on the diversity of device design, device performance, and ultimately overall system performance.
It is the objective of the present invention to remove these constraints by providing bulk ternary and quaternary base nitride substrates whose composition, and hence lattice constants, can be tuned to match any desirable composition(s) of over-layer structures for any device structure grown on them. It is further the object of the invention to provide an engineering flexibility that can accommodate small degrees of mismatches elastically without generation of dislocations or defects. Such an elastic accommodation can in fact be used to provide an additional degree of freedom of design.